The problem of storage of nuclear waste products from both military and civilian sources is presently becoming so acute that further progress, particularly in the field of development of nuclear energy, is threatened. The solutions proposed thus far generally have been based on irretrievable disposal of the wastes. The problems and uncertainties associated with such disposal are clearly summarized in Geological Survey Circular 779, entitled "Geologic Disposal of High-Level Radioactive Wastes . . . Earth-Science Perspectives," by J. B. Bredehoeft et al. Accordingly to these authors, it is estimated that 476,000 spent fuel assemblies will be on hand by the year 2000, occupying 3000 m.sup.3, if processed as high-level waste, and an order of magnitude more in intermediate-level waste. The toxicity of these wastes is illustrated by the fact that the quantity of water that would be required to dilute the wastes on hand by year 2000 to levels considered safe is double the quantity of fresh water in global storage. The urgency of the problem is illustrated by the discovery that increasing levels of artificial radionuclides in Lake Erie and Ontario have been traced to Cattaraugus Creek, which flows by an interim waste-storage facility at West Valley, N.Y. (Industrial Research/Development, July 1978, page 46).
The size of the space necessary to accommodate such waste would be only about 10 km.sup.3, but the penalty for error in selecting the placement and planning of the facilities to provide it would be most severe. Critical features to be considered are small faults or fracture systems, which are extremely difficult to detect. Although techniques for non-destructive characterization of the site have recently been proposed, these systems require further refinement.
In addition to the problems that arise from the nature of the site prior to storage of the waste products therein, the effect of the mechanical, chemical and thermal disturbance arising from the waste products themselves must be provided for. Canisters of high-level waste may produce 5 kW of heat ten years after reprocessing and it may take as much as one hundred years for the rate to decrease to 0.5 kW. No adequate model of the effects of the hundred-year thermal output exists.
The form of the waste during storage is, of course, of crucial importance. At the present time it appears to be well accepted that high-level wastes from reprocessing may be cast in the form of glass billets, the glass having a very low leachability (Marsily et al, 1977, Energy Research and Development Administration 1976, Page 7). Nevertheless, some contaminants will be released and the nature of the glass itself may change as a result of the radiation so that the rate of leaching will undoubtedly change with time.
Large gaps exist in our knowledge of transport systems, that is, with respect to the course by which released contaminants may reach the biosphere. The problems in this area include lack of knowledge of fractures, natural or manmade, and of measurements of the effect of fluid head and permeability of the rock or salt within which the billets are stored. Data are largely unavailable so that the complete description of groundwater flow is a problem still awaiting solution.
The goal, of course, is to contain the waste and prevent it from reaching the biosphere until it is no longer hazardous. Estimates have been made as to the length of time for which the waste must be stored so that it presents no further damage. Strontium 90 and cesium 137 will constitute 99% of the projected curie accumulation by the year 2020, but these will be reduced to one millionth the initial radioactivity in 600 years. However, the toxicity from iodine 129 and radium and the actinide elements will remain well over ten million years. Ferruccio Gera, 1975, "Geochemical Behavior of Long-Lived Radioactive Wastes: U.S. Energy Research and Development Administration, Oak Ridge National Laboratory, report ORNL TM-4481," at page 14 considered that assuring containment for longer than five million years is "clearly impractical since totally reliable geologic predictions of the detail required over such long time frames are beyond present capability."
The science of geologic prediction is limited by the assumption of constancy of rates of processes and incomplete data; the data in U.S. seismic records go back for only two hundred years. Accordingly, "validating a waste-management model for the time spans of concern will never be possible," according to this author. Even routine predictions of one-hundred-year processes have varied from good to poor. Predictive models, an essential step in selecting a site and managing the waste, have components that are inherently unpredictable at present. Accordingly, these models will not give a single answer to the fate of radioactive waste in geologic repositories but rather a spectrum of alternative outcomes based on uncertain assumptions about the future.
As aforenoted, present thinking is based on the concept of incorporating the nuclear waste in glass and then irretrievably storing the glass, as billets, in a suitable storage space, a substantial thickness of earth providing the shielding. In this concept the sole defense against the access of water, which could solubilize the radioactive material, leading to its transport into the biosphere, is the choice of a stable site. However, as discoveries of the last few years have emphasized, the earth's crust is not stationary. Earthquakes have been noted throughout recorded history, and are much more frequent along certain well-known fault lines, but severe earthquakes in previously "immune" regions are not unknown. More important is the fact that the surface of the earth consists of so-called tectonic plates which move about on the surface of the melt below and clash with each other. The collision between the plates gives rise to mountain ranges and to other types of severe deformation of the crust. Such actions take place both slowly over geologic times and abruptly. Unfortunately, the period over which the nuclear waste must be safeguarded is comparable with the geologic periods over which even the slow but severe changes in the shape of the earth crust may take place. The time of ten million years has already been mentioned above. It has been pointed out that improved reprocessing could reduce the period of concern to about 1000 years; this would reduce but not eliminate the uncertainties. To project the safety of high-level radioactive wastes irretrievably stored over such time periods is therefore impossible. Nevertheless, proposals continue to be put forward that call for storage of such glass billets in hard rock caverns or in salt mines which obviously have been free of ground water for long periods of time. Such proposals originate not only in the United States but in other countries as well. For instance, Frank Feates and Norman Keen of the technology division of Atomic Energy Research Establishment, Harwell, England, publishing in the New Scientist of Feb. 16, 1978, propose that liquid waste be converted into glass and encased in steel cylinders 60 cm in diameter and about 3 meters long, each containing about 1.4 tons of glass. They consider that the glass would probably require cooling for several years before final disposal. As the repository for the cylinders, emplacement on or under the ocean bed appears to be a very safe method, according to these authors. They also view favorably the study of disposal in salt formations or clay or hard rock formations, suggesting that the repository be at least 300 m deep so as to lie below the permafrost level in any future ice age. Even so, they do not propose that disposal be carried out at this time, but rather that further data be accumulated. They suggest that small bore holes be drilled so as to investigate fracture structure using television and various physical techniques.
In addition to the problems of storage, there are beginning to be difficulties in the above-ground transport of nuclear wastes. Perceived dangers have led to laws and ordinances restricting passage of vehicles carrying radioactive cargo, for example, in New York, New Jersey and Connecticut (New York Times, Apr. 17, 1978).
A number of writers have also raised the concern that fissile isotopes (uranium 235 and plutonium) from nuclear power cycles may, because of their usability in nuclear weapons, become targets for terrorists or blackmailers. This imposes the additional requirement of strict accountability for the wastes during disposal, a condition difficult to satisfy when large numbers of billets must be handled.
Methods of preparing fused glass bodies and suitable compositions have received considerable attention, J. R. Grover et al in U.S. Pat. No. 3,321,409 proposing to mix a radioactive waste liquid with a dry powder in a container, driving off the water and heating the product to fusion to form a glass. Joseph Kivel et al in U.S. Pat. No. 3,364,148 manufacture a source of radioactive energy by enclosing an insoluble radioactive material in a heat-fusible, continuous matrix comprising at least 92% by weight of silica, the peripheral portion of the matrix being free of the radioactive material. H. D. Bixby in U.S. Pat. No. 3,249,551 teaches the disposal of high-level radioactive waste materials by mixing the waste materials in clay and firing the mixture to make a ceramic body; the ceramic body is then covered with a ceramic glaze. F. C. Arrance in U.S. Pat. No. 3,093,593 disposes of radioactive waste by mixing same with ceramic materials, adding water to the mixture, shaping into porous pieces, pre-firing the pieces to destroy the ion-exchange capacity of the ceramic materials, saturating the pieces with radioactive waste materials by absorption, drying and finally firing.
Kuan-Han Sun et al describe the preparation of a radioactive fluophosphate glass composition and making glass fibers of same, in their U.S. Pat. No. 3,373,116. According to the inventors, the glass may be used either in the form of thin glass fibers or small glass particles as a fuel for nuclear reactors. W. W. Schulz et al U.S. Pat. No. 4,020,004 describe the manufacture of a borosilicate glass incorporating radioactive cesium. Werner Hild et al in U.S. Pat. No. 3,971,717 propose to make solid glass blocks containing radioactive wastes and then to place them in water in order to condition the water, said conditioning including sterilization and facilitation of the filterability of the sludge.
As is evident from the above, the incorporation of radioactive waste materials in a glass, particularly of the borosilicate type, has received considerable attention, based, apparently, on the belief that the glass is essentially unattackable by ground water; it is still an open question whether glass is actually impervious to attack by water over period of time extending to millions of years. In fact, accelerated tests at high pressures and temperatures by G. J. McCarthy et al indicate, to the contrary, that such glass is subject, first, to discoloration and fissure formation and, finally, to fracture and crystallization (Chemical and Engineering News, June 1, 1978, page 28). In addition, the concept of storage under ground is also looked upon with more or less favor but, as mentioned above, it is beginning to be realized that the integrity of the storage region cannot be predicted with any degree of certainty, so that this solution to the problem of waste disposal must be regarded as flawed. Other modes of disposal, such as firing the waste into solar orbit have also been proposed, but these are not economically feasible at this time. It is evident that either a new approach or an improved approach, eliminating the aforenoted problems, is needed.